Loading pipe in a cargo pressure tank of a ship

ABSTRACT

A device for a pipe ( 10 ) in a cargo pressure tank of a ship ( 2 ), the pipe ( 10 ) mainly being used for loading and unloading of fluids, and the open end portion ( 13 ) of the pipe ( 10 ) being placed immediately above the lower bottom section ( 14 ) of the cargo pressure tank ( 1 ).

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/NO03/00215, filed Jun. 26, 2003, whichinternational application was published on Jan. 14, 2004 asInternational Publication WO 2004/005790. The International Applicationclaims priority of Norwegian Patent Application 20023256, filed Jul. 4,2002.

BACKGROUND OF THE INVENTION

This invention regards a loading pipe in the cargo pressure tank of aship. In particular, it concerns a pipe placed inside a cargo pressuretank, wherein the pipe is arranged especially for use during loading andunloading of the ship.

Sea transport of gaseous petroleum products has taken place mainly bymeans of the so-called Liquefied Natural Gas (LNG) method. The methodincludes cooling of gas to a liquid state, whereupon the gas may betransported in ship tanks at atmospheric pressure. Costly equipment isrequired both at the location of shipment and at the receiving end. Asthe gas must be cooled to a relatively low temperature, up to a fifth ofthe gas is used for operation of the cooling and heating processes.Consuming this amount of energy solely for transport-related processesis expensive and also environmentally questionable.

Several other ship-based solutions have been proposed, in which the gasis pressurised and/or cooled in order to attain a gas density that ispractical for the purpose. Such solutions have gained little practicaluse, but a solution in which a large number of vertical tubular pressuretanks are placed in the cargo hold of a ship, has attracted considerableattention. The method is termed Pressurised Natural Gas—PNG. Accordingto such a method, the gas is compressed at the shipment site to anoverpressure of a couple of hundred bars, and then is filled onto thecargo pressure tanks located on the ship. The cooling is limited to asimple and inexpensive removal of the compression heat from the gas, thetransport temperature thus becoming close to the ambient temperature.

When using a relatively large number of preferably vertically mountedcargo pressure tanks in the hold of the ship, it is desirable for eachcargo pressure tank to have as few pipe connections as possible. Pipeconnections subjected to high pressure during loading, transport andunloading should be monitored continuously.

Essentially, the pipework of the ship must allow complete emptying ofthe cargo pressure tank. Moreover, it must be designed so as to alloweasy inspection of the external pipe connections on the cargo pressuretank.

SUMMARY OF THE INVENTION

The object of the invention is to describe a device in which it ispossible, by means of a singular pipe connection, to meet theabove-mentioned characteristics.

The object is achieved in accordance with the invention by means offeatures disclosed in the description below and in the subsequent patentclaims.

Prior to finishing the top section of the cargo pressure tank andinseparably connecting it to the cylindrical mid-section of the cargopressure tank, a pipe is placed inside the cargo pressure tank. The pipeprojects from just above the inner bottom section of the cargo pressuretank and preferably up to the upper section of the cargo pressure tankwherein the pipe, via a bushing through the wall of the cargo pressuretank, is connected to the external pipe connection of the cargo pressuretank.

In at least one position along its longitudinal extent, the pipe isconnected to a guide displaceable against the internal wall of the cargopressure tank.

Preferably suspended from the upper section of the cargo pressure tank,the pipe thus may expand freely in its longitudinal direction throughdisplacement in the longitudinal direction of the pipe. In order toprevent mutual, relative movement between the cargo pressure tank andthe pipe in the radial direction of the pipe, and to dampen anyvibrations that might occur in the pipe, the guides preferably arelightly pretensioned against the internal wall of the cargo pressuretank.

During loading, fluid flows down through the pipe to the bottom sectionof the cargo pressure tank, whereby the pressure in the cargo pressuretank increases. When the cargo pressure tank is filled with petroleumgases, for example, only a negligible part of the cargo volume in thecargo pressure tank will be in gas phase due to the relatively highpressure.

During unloading from the ship, if a portion of the gases condensed inthe cargo pressure tanks during loading does not boil off again, therebyfollowing the liquid phase out of the cargo pressure tank, insufficientpressure remains in the cargo pressure tank to drive all of the cargoliquid phase up through the pipe. This may be remedied by pumpingpressurised gas down through the pipe. The pressure in the cargopressure tank thereby is sufficiently increased to drive the remainingliquid phase out before the remaining gas then flows out until thepressure in the cargo pressure tank is approximately equal to thedesired pressure.

If desired, the pipe guide(s) may be fixedly attached to the cargopressure tank, while the pipe may be moved in the guide.

Essentially, the pipe suspended in the cargo pressure tank is subjectedto the load of its own weight only. Moreover, a relatively moderatepressure difference will exist between the inside and the outside of thepipe. The cargo density and the height of the cargo pressure vesseldetermine the pressure difference. Thus, the pipe may be provided withrelatively small material thickness, as the pipe is braced by at leastone guide.

BRIEF DESCRIPTION OF THE DRAWINGS

The following describes a non-limiting example of a preferred embodimentillustrated in the accompanying drawings, in which:

FIG. 1 schematically shows a section through a larger number of cargopressure tanks installed in a hold of a ship;

FIG. 2 shows a longitudinal section through a cargo pressure tank; and

FIG. 3 shows a section II—II in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, reference number 1 denotes a cargo pressure tank thattogether with other cargo pressure tanks 1, is arranged upright in ahold 4 of a ship 2. The cargo pressure tanks 1 are connectedindividually or in groups to the main loading and unloading pipe 6 ofthe ship via connecting pipes 8.

A pipe 10 projects downward from the upper section 12 of the cargopressure tank 1, and its opening 13 is placed immediately above thelower section 14 of the cargo pressure tank 1. The pipe is sealinglyconducted through an opening 16 in the upper section 12 of the cargopressure tank 1 and is connected to a pipe coupling 18, shown in theform of a pipe flange herein.

The upper and the lower sections 12, 14 of the cargo pressure tank 1 areinseparably connected to the cylinder body 15 of the cargo pressure tank1.

The pipe coupling 18 is sealingly connected to one of the connectingpipes 8.

In at least one position along its longitudinal extent, the pipe 10 isprovided with a substantially radially projecting guide 20. The slidingsurfaces 22 of the guide 20 bear displaceably against the internal wall24 of the cargo pressure tank 1.

The pipe 10, which is thus suspended from and connected to the cargopressure tank 1, is arranged to allow it to essentially expand freely inits longitudinal direction, simultaneously being prevented from movingin the radial direction by the guide 20.

Thus the pipe 10 is subjected to relatively small mechanical loads andmay be of a slender construction.

Loading and unloading of the cargo pressure tank 1 by means of the pipe10 is carried out in a manner described in the general part of thedescription.

The guide 10 may have alternative geometric designs, for example aspherical shape, and being provided with, for example, a damper materialon the sliding surfaces 22.

1. A cargo pressure tank for the transport of petroleum products, the cargo pressure tank comprising: a tank body having an upper end and a lower end; an elongated pipe disposed in the tank body, the pipe having its first open end positioned adjacent the lower end of the tank body and its second open end sealingly engaged with the upper end of the tank body; wherein the pipe is arranged to facilitate loading and unloading of pressurized natural gas in either a liquid or nearly liquid state into and out of the cargo pressure tank; wherein the pipe is provided with at least one guide, the guide being arranged to reduce displacement of the pipe in the radial direction, and wherein the guide is located between the upper and lower ends of the tank body and is displaceable in the longitudinal direction of the cargo pressure tank.
 2. The cargo pressure tank according to claim 1, wherein the pipe extends from and is suspended from the top section of the cargo pressure tank.
 3. The cargo pressure tank according to claim 1, wherein the second open end of the pipe is attached to the uppermost part of the tank body.
 4. The cargo pressure tank of claim 1, wherein the guide is pretensioned against an internal wall of the cargo pressure tank.
 5. The cargo pressure tank of claim 1, wherein the guide comprises sliding surfaces for contacting an internal wall of the cargo pressure tank.
 6. The cargo pressure tank of claim 5, wherein the sliding surfaces comprise a damper material.
 7. A cargo pressure tank for the transport of petroleum products, the cargo pressure tank comprising: a tank body having an upper end and a lower end; an elongated pipe disposed in the tank body, the pipe having its first open end position adjacent the lower end of the tank body and its second open end sealingly engaged with the upper end of the tank body; wherein the pipe comprises the only connection between the outside and inside of the cargo pressure tank, and is arranged to load and unload pressurized natural gas in either a liquid or nearly liquid state into and out of the cargo pressure tank; at least one guide disposed in the tank body and arranged to reduce displacement of the pipe in the radial direction; wherein the guide comprises sliding surfaces for contacting an internal wall of the cargo pressure tank.
 8. The cargo pressure tank of claim 7, wherein the sliding surfaces comprise a damper material. 